1. Field of the Invention
The present invention relates to an organic electroluminescent device (luminescent device or EL device) that emits light by converting electric energy to light.
2. Description of the Related Art
Today, research and development on various display devices are being vigorously conducted. Among these, organic electroluminescent (EL) devices have attracted attention as promising display devices since light emission can be obtained with high luminance at low voltage.
Generally, organic EL devices have a pair of electrodes and a luminescent layer or a plurality of organic layers including a luminescent layer that are disposed between the electrodes, and emit light with the excitons generated by recombination of the electrons injected from a cathode and holes injected from an anode in the luminescent layer or with the excitons of other molecules generated by energy transfer from the excitons above.
The organic EL device is a self-luminous surface-light source, and may be used, for example, as a white light source. An ideal white light source has a coordinate pair (0.33, 0.33), as defined by Commission Internationale d'Eclairage (CIE). White light may be obtained based on the combination of luminescent materials in three colors of blue, green, and red. Alternatively, white light may be obtained based on the combination of luminescent materials in two complementary colors.
As a white luminescent device, one which emits white light with high luminance and high chromaticity at low voltage is desired. Use of a phosphorescent material, which has higher luminous efficiency than a fluorescent material, is preferable for reduction of operation voltage and improvement in luminance (see e.g., Japanese Patent Application Laid-Open (JP-A) Nos. 2001-319780, 2004-281087 and 2004-522276, the disclosures of which are incorporated by reference herein). In particular, there has been a need for development of a blue phosphorescent material and a device that emits light effectively from the blue phosphorescent material, for improvement in the luminous efficiency of luminescent devices. This is because, if the intensity of blue light is low, in order to obtain a light of desired chromaticity, it is necessary to lower and adjust the intensities of green and red phosphorescences that are known as being emitted at high efficiency, resulting in decrease in the luminous efficiency of the resulting luminescent device. Thus, conventional devices are still unsatisfactory in luminous efficiency and chromaticity, and further improvement is needed.